Just got the Brain Surgeon trophy as well. Its a lot easier if you just host a multiplayer match with novice bots and add the Slow Mo Mutator so that you can increase your reaction time and thus increase your aim. It takes about 15 headshots in a round with the sniper rifle to get the Head Hunter award. The bots usually will stand still and shoot at you from a distance as well. Perfect for sniping their heads clean off their shoulders.